I did a quick search and didn't see this issue addressed, so I thought I would pass this along.
I was in one of my LGS's yesterday, and while I was there I looked at their stock of Glocks. I asked one of the sales guys about impressions of the Gen4's. He said I should talk to Travis, another sales guy, who just went through Glock Armorer training. So I went and asked Travis about it, and he got a frown on his face and told me the quality is going downhill. Apparently, Glock is starting to use some mim parts in their Gen4's, and that they are seeing more Glocks coming in for repairs than they used to. He also said that some of Glock's replacement parts will soon also be mim for all models, not just for Gen4's. I felt a little better when he said that Glock will continue to make Gen3s concurrently with the Gen4's, and that there is no plan to discontinue the Gen3's at all. My good feeling didn't last long though, because he told me that they will switch over to mim parts for the Gen3's as well.
I was pretty bummed, as I was looking forward to picking up a Gen4 G20 within the next few months. The main reason I was looking for a Gen4 is because of the interchangeable backstraps. So, I guess I'll be looking for a Gen3 G20 SF instead. The real bummer of it all is we won't be able to get decent replacement parts any more after they complete the switchover to mim.